Nonaqueous electrolyte secondary batteries, especially lithium-ion secondary batteries, each have a high energy density. Thus, such nonaqueous electrolyte secondary batteries have been widely used as batteries for use in, for example, a personal computer, a mobile phone, and a portable information terminal, and also have recently been developed as on-vehicle batteries.
Conventionally, a microporous film that contains polyolefin as a main component has been used as a nonaqueous electrolyte secondary battery separator (hereinafter may also be referred to as merely a “separator”) included in a nonaqueous electrolyte secondary battery such as a lithium-ion secondary battery (Patent Literature 1).
The microporous film has therein pores connected to one another, and allows a liquid containing ions to pass therethrough from one surface to the other. This microporous film is thus suitable as a battery separator member that exchanges ions between a cathode and an anode.
Meanwhile, recent widespread use of nonaqueous electrolyte secondary batteries has created a demand for longer-life nonaqueous electrolyte secondary batteries.